Official ISO downloads: https://www.microsoft.com/en-us/software-download/windows10I...
Presumably you have to do an in-place upgrade if you use these? I can't see any mention of what key I am supposed to use (from the reservation tool) if I want to do a fresh install.
You can then use that to do a clean install.
EDIT
I've just read contradictory information here;
http://windows.microsoft.com/en-gb/windows-10/media-creation...
If you upgraded to Windows 10 on this PC by taking advantage of the free upgrade offer and successfully activated Windows 10 on this PC in the past, you won't have a Windows 10 product key, and you can skip the product key page by selecting the Skip button. Your PC will activate online automatically so long as the same edition of Windows 10 was successfully activated on this PC by using the free Windows 10 upgrade offer.
Which makes it sound like the version of Windows 10 you received as a free upgrade from Windows 7/8 will not work on any other machine than the one you first install on.
Which is unbelievably shit.